Molecular states near a collision threshold

This chapter summarizes the properties of near-threshold molecular states that can be formed from ultracold atoms. The properties of the states for a single potential are characterized for different partial waves using the concepts of quantum defect theory for a van der Waals potential. Examples are given for various isotopes of the Yb_2 molecule. Magnetically tunable Feshbach resonances or photoassociation resonances are characterized by the multichannel version of the theory. Examples of the latter are given for the KRb molecule.